The present invention relates to a signal recording/reproducing method for correcting the interval in a series of reproduced data obtained from read-out signals of record data recorded in a recording medium to obtain a normal, interval and an apparatus for realizing same. The invention relates in particular to a signal recording/reproducing method capable of reproducing data stably also against variations in the length of data (pit length) taking place at the time of recording data on an optical recording medium by utilizing heat of a light beam and an apparatus for realizing same, which are suitable for use in an optical disc storage apparatus.
In a prior art optical disc storage apparatus, as disclosed e.g. in U.S. Pat. No. 4,835,759 (JP-A-Sho 62-8370), a method is used, by which, in order to absorb or correct variations in the length of recording pits produced by variations in the characteristics of the recording medium and recording conditions, a predetermined demodulation start-pattern, i.e. SYNC pattern, is made to correspond to leading-edges and tailing-edges of the recording pits, as means for detecting the amount of variations from the normal length of the pit, and a time difference between the 2 SYNC pattern detection signals obtained from the leading-edges and the tailing-edges at the time of reproduction is detected by a time-difference detecting circuit consisting of a delaying element having a plurality of taps with a constant delay time interval and flipflops. By this method, since the time-difference the detection amount was determined by the time between two adjacent taps of the delaying element, results were dispersive and there were problems in the detecting resolution and the detecting accuracy. Further, apart from the fact that it is difficult to suppress fluctuations among different apparatuses to small values, even for a certain optical disc storage apparatus, since the detection and the correction of the variations in the pit length are effected by using only the heading of each sector, it is thought that, in the case where media-characteristics and recording conditions are different for the first half and the second half of the sector, there are problems that errors are great and that the overhead becomes somewhat greater, because a duplex SYNC pattern is used.
On the other hand, as a method for correcting variation in the pit length at the reproduction, a method proposed in JP-A-Sho 62-254514 is known. This is a method, by which an edge detecting circuit, a synchronizing clock generating circuit, etc. are added to a threshold setting circuit, and a reproduction clock signal syhchronized with the leading-edges of reproduced pulses made binary at a certain threshold value is generated by the clock generating circuit, the reproduced pulses being synchronized again by using this reproduced clock. Here a difference signal between the original reproduced pulse and the resynchronized reproduced pulse is formed and deviations of the tailing-edges of the reproduced pulse with respect to the reproduced clock are detected. This difference signal is given to the threshold setting circuit through a low pass filter. In this way it is possible to obtain the optimum threshold value by increasing or decreasing the threshold value so that the difference signal is zero.
By this method, the least condition for demodulating data precisely is that variations in the position of the tailing-edges with respect to the leading-edges of the reproduced signal are comprised within a distance corresponding to the time width of one period of the reproduced clock. Since one period of the reproduced clock corresponds in general to the data detection-window width, with other words variations in the position of the pit edges allowed at data recording should be necessarily comprised within the detection-window width. However, in the practical recording state, it is conceivable that variations exceeding the detection-window width are produced by various conditions, such as variations in the recording power and the recording pulse width, sensitivity fluctuations of the recording medium, variations in the intensity distribution of the recording/reproducing light spot, etc. There is a possibility that these variations take place not only in ablative recording media, but also in phase-change media, magneto-optical media, etc. In any case, in the case where recording portions (recording pits or recording domains) exceeding the detection-window width are formed, unless some correction is executed, erroneous data is recognized. A method may be conceivable by which the threshold value is increased or decreased, relying upon the duty ratio of the data pattern so as to obtain the normal data pulse width. However, unless the gradient of the leading-edges and the tailing-edges of the reproduced signal is sufficiently small, it is not possible to correct completely the errors only by the increase or decrease of the threshold value. On the contrary, if the gradient is too small, it is feared that variations in the position of the edges of the reproduced pulse after having been made binary are great due to noises contained in the reproduced signal. In a prior art circuit for producing binary data, for the reason described above, the correction is not possible for variations at the recording exceeding the detection-window width.
As described above, according to the prior art technique, in the case where variations in the recorded pit length produced by variations in the recording condition at the recording exceeded the detection-window width, there was a problem that the correction could not be executed only by increasing or decreasing the threshold value at the time of producing binary data.
Further, an information recording/reproducing method capable of recording/reproducing information in a stable manner by using edge record recording coded bits of the data, making them correspond to the leading-edges and the trailing-edges of the recording pits, and correcting the recording pulse width and the recording power at the time of recording so as to correct the variations in the edge position at the reproduction, has been proposed in U.S. Pat. No. 4,866,692.